A known type of piezoelectric motor comprises a slab of piezoelectric material having a common electrode on one surface and two discrete electrodes on the opposite surface. When the common electrode conducts with one of the discrete electrodes the piezoelectric motor moves in one direction. When the common electrode conducts with the other discrete electrode the piezoelectric motor moves in the opposite direction.
FIG. 6 shows a prior art drive circuit connected between the common electrode and one of the discrete electrodes of a piezoelectric motor. The drive circuit comprises a direct current power source DC, two half bridge circuits HB1,HB2, an inductance L and a piezoelectric motor M connected in series. Each half bridge circuit comprises two switches controlled by PWM signals. In one half cycle switches S1 and S4 are controlled to conduct while switches S2 and S3 are controlled to non-conduct and current passes through the piezoelectric motor M in a direction from node n1 to node n2. In the other half cycle switch S1 and S4 are controlled to non-conduct and switches S2 and S3 are controlled to conduct and current passes through the piezoelectric motor M in a direction from node n2 to node n1. The piezoelectric motor M is capable of moving in one direction when alternating current passes there through.
However, the above known drive circuit requires two half bridge circuits each consisting of two switches corresponding to movement of the motor in one direction only. At least one additional half bridge circuit comprising two switches is needed for movement of the motor in two directions, which is expensive.